what means nTargetTimespan or difficulty re-target?

[supercanada1]

what means nTargetTimespan or difficulty re-target? any definition?

[achow101_alt]

The difficulty retarget is when the difficulty of the network is changed every 2016 blocks so that there is a 10 minute interval between each block. This retarget should occur roughly every 2 weeks, which is the timespan for the retarget.